<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            height: 2000px;
        }
        .pos{
            position: fixed;
            bottom: 100px;
            right: 100px;
        }
    </style>
</head>
<body>
    <h1>hello，world</h1>
    <h1>hello，world</h1>
    <h1>hello，world</h1>
    <h1>hello，world</h1>
    <h1>hello，world</h1>
    <h1>hello，world</h1>
    <button class="pos"><a href="#">返回顶部</a></button>
</body>
</html>